Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2151105
Method for makinq a qas diffusion electrode.
Method for making a gas diffusion electrode, whereby at
least one layer of a liquid containing active particles
and a binding agent in a solvent is sprinkled or atomized
on a porous, hydrophobic support.
Such methods are applied for making electrodes for
electrochemical cells such as fuel cells, whereby the
layer is a catalytic layer and the active particles are
catalyst particles.
In the same way can also be provided an intermediate
layer between the support and the catalytic layer, in
which case the active particles are electron conductive
- particles.
In "Nouveau type d'électrodes pour piles à combustible
~ hydrogène/oxygène" of S. Escribano, R. Mosdale and P.
Aldebert, introduced at the symposium "Réalités et
Perspectives du Véhicule Electrique" in La Rochelle
(France) of 15-19 November 1993, such a method i6
- mentioned whereby, on a support of porous carbon,- a
solution of an electron conductor and PTFE i6 atomized
first, and subsequently a solution of a catalyst, an
electron conductor and PTFE. In between atomizations, the
solvent is evaporated. Afterwards, a layer of NAFIONR;is
atomized on the whole.
Besides, the sprinkling or atomizing of a solution or
suspension of solid particles on a support is a
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conventional technology. It allows to apply thin and
relatively homogenous layers.
The evaporation of the solution after the sprinkling or
atomization usually takes place in the air, but it can be
speeded up by heating, for example by putting the support
with the provided layer in an oven and by heating it.
The problem with these known methods is to simultaneously
obtain the required porosity and firmness of the layer
provided on the support.
The invention aims to remedy this problem and to provide
a method for making a gas diffusion electrode which makes
it possible to provide a layer on a support which is
porous and whose particles are firmly bound to one
another.
To this aim, the sprinkling or atomization is carried out
by adding heat to the sprinkled or atomized liquid, so
that the solvent is partly evaporated before the support
is reached.
In fact, a sort of spray-drying is to a certain degree
obtained in this way.
By partly evaporating the solution, the active particles
are surrounded by a film of the binding agent before they
reach the support. These coated particles are put
against one another as individual granules and, as the
binding agent has not dried entirely yet, are glued
together by this binding agent. In this manner, the
active particles are homogeneously distributed and evenly
surrounded by the binding agent.
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-
The heat supply can be obtained by sprinkling or
atomizing in a hot environment or by directly heating the
sprinkled or atomized liquid, for example by means of
infrared rays.
In a practical embodiment of the invention, a liquid is
sprinkled or atomized which has catalyst particles as
active particles.
For many applications, platinum is the most suited
catalyst, and the catalyst particles are platinized
carbon particles.
According to another practical embodiment of the
invention, a liquid is sprinkled or atomized with an
ionomer as a binding agent.
It was found that for electrodes, the use of an ionomer
_ instead of a binding agent such as PTFE provides a better
result.
~ A suited ionomer is NAFIONR, which is a perfluoric
sulphonic acid commercialized by Du Pont de Nemours.
The support can be either or not permanent. In case the
- support is permanently part of the electrode, it is of
course porous and hydrophobic. A suited support is a
support obtained by applying a paste of electrically
conductive material, in particular carbon and a binding
agent, in particular PTFE, on a fabric such as a carbon
fabric.
Other particularities and advantages of the invention
will become clear from the following description of a
.,, 2I5IIo~
-
method for making a gas diffusion electrode according to
the invention. This description is given as an example
only and does not restrict the invention in any way. The
figures refer to the accompanying drawings, in which:
Figure 1 shows a device with which the method
according to the invention is applied;
figure 2 shows a device analogous to that in figure
1, but with which can be carried out another
embodiment of the method according to the invention.
The figures represent a device with which a catalyst
layer can be provided on a support to make an electrode
according to the invention.
According to figure 1, this device contains a rotating
table 1 which is mounted on the frame 2 and i8 driven by
an electric motor 3. Above the table 1, but
eccentrically in relation to the axis of rotation, is
situated the sprinkler head 4 of a device 5 for
sprinkling or atomizing, which is mounted on the frame 2.
This device 5 can be of a know construction and is not
described in detail here.
What is important is that, next to the sprinkler head 4,
is mounted an infrared lamp 6 which directs infrared rays
to the sprinkled liquid.
This liquid is a suspension of ink 7 which consists of
active particles mixed with a binding agent, in
particular an ionomer, in a solvent.
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-
To make an electrode, one proceeds as follows:
A support 8 is placed on the table 1. If it is
permanently part of the electrode, this support must also
be porous and electrically conductive. A suited support
is a support consisting of a carbon fabric upon which was
provided a mixture of carbon and a binding agent such as
PTFE and then dried.
Subsequently, the above-mentioned ink 7 is sprinkled on
said support while the infrared lamp 6 is working. Due
to the heat of this lamp, the solvent in the small liquid
drops will partly evaporate before these drops reach the
support 8.
As a result, granules of active particles are formed even
in the air, surrounded by a binding agent which has not
entirely dried yet, and a granular, porous layer is
_ obtained on the support 8 in which the granules firmly
stick together due to the binding agent. The binding
agent and active particles are evenly distributed and all
- active particles are surrounded by a binding agent, so
that, after further drying, a very firm, porous layer is
obtained.
- This method is particularly suited for making a catal-yst
layer whereby the active particles are catalyst
particles, for example platinized carbon, and the binding
agent is preferably an ionomer. This layer can be
sprinkled in one or several stages, whereby a furt~er
evaporation of the solvent is possible in between said
stages. The layer can have a thickness of a few
micrometers, and the amount of catalyst can be situated
between 0.10 and 0.50 mg Pt/cm2. In case platinized
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carbon is used as catalyst particles, these particles can
contain for example 10 to 40 percentage by weight of
platinum.
In an analogous manner can also be made a condùctive non-
catalytic intermediate layer whereby the active particles
are electron conductive particles, such as carbon
particles, instead of catalyst particles, and the binding
agent is preferably also an ionomer. Also this layer,
which was sprinkled in one or several stagesj can be a
few micrometres thick.
The device represented in figure 2 makes it possible to
apply the same principle, whereby the solution partly
evaporates before the ink 7 reaches the support 8, but
the evaporation is not caused by direct radiation of an
infrared lamp 6, but because the sprinkling takes place
in a closed, heated environment or space 9 from which the
solvent vapours are extracted via a duct 10.
Also in this device is provided a stationary mounted
sprinkler device 4 in the above-mentioned space 9, above
a heated endless belt 11 which is moved over rollers 12
and upon which the support 8 is placed. The space 9 is
confined by the endless belt 11 and a cap 13 mounted
above it to the top of which the duct 10 is connected.
The space 9 is heated by means of hot air or by means of
an infrared lamp 6 mounted in it, whereby the heating of
the endless belt 11 also contributes to this heating.
In both embodiments, the evaporation of the solvent
depends on the intensity of the heating which can be set
such that, for a given binding agent, the optimal grain
formation and adhesive power between the granules is
21 511 D5
obtained.
The obtained porous gas diffusion electrodes which are
made in this way are very homogenous and have excellent
mechanical and electrochemical qualities. This method is
particularly suited for making electrodes, in particular
cathodes for fuel cells, whereby for example a non-
catalytic intermediate layer is provided on a porous
support with a catalyst layer on top of it.
Above the rotating table 1 of the endless belt 11 can be
provided several sprinkler devices 5, so that several
identical or different layers can be sprinkled one after
the other.
